Final answer:
An antigen-presenting cell initiates an adaptive immune response by processing and presenting viral antigens bound to MHC Class I molecules, which are then recognized by CD8 T cells.
Step-by-step explanation:
The mechanism best describing how an antigen-presenting cell (APC) triggers an adaptive immune response, particularly for a cell infected with a virus, involves a complex process called antigen processing and presentation.
This process starts with APCs, such as dendritic cells, macrophages, or B cells, internalizing pathogens and breaking them down enzymatically into smaller antigenic fragments. These fragments are then presented on the cell surface bound to major histocompatibility complex (MHC) molecules. For virus-infected cells, the antigens are typically presented with MHC Class I molecules, which are recognized by CD8 T cells, also known as cytotoxic T cells.
Clonal expansion and selection are critical for amplifying the immune response. Once an APC presents an antigen to a T cell, and if the T cell receptor recognizes the antigen-MHC complex, the T cell becomes activated. This leads to mitotic division of the T cell, producing a larger number of T cells specialized against the specific pathogen. The specificity of the T cell response is based on the unique T cell receptors that recognize the antigen-MHC complex.
There is also a process called cross-presentation used when intracellular pathogens do not directly infect APCs. In this case, dendritic cells can present extracellular antigens on MHC Class I molecules, activating CD8 T cells in a similar manner as if the dendritic cells themselves were infected. Overall, the recognition of antigen-MHC complexes by T cells initiates a series of events that lead to the activation and expansion of specific T cells to fight off the infection.